Well, the thing that sets living stuff apart from non-living seems to be the ability to sustain life (usually in the form of offspring). From this we can infer the purpose of life to be keeping life alive.
Spreading live across the universe being a good way to keep some of it going seems plausible. I'd obviously prefer intelligent beings like humans doing so, since so far evolution indicates this development path to have the best chances long term. But if not possible or too costly, spreading the next closest thing to us that is possible makes sense. At least as a form of backup. And sending microbes seem quite easy...
Spreading live across the universe being a good way to keep some of it going seems plausible. I'd obviously prefer intelligent beings like humans doing so, since so far evolution indicates this development path to have the best chances long term. But if not possible or too costly, spreading the next closest thing to us that is possible makes sense. At least as a form of backup. And sending microbes seem quite easy...